It was only three years ago that Aaron Patzer started Mint.com, a free web-based financial planning site. In his mid 20s, he encountered much skepticism. But, this was no problem for Aaron. He realized he could make a difference.

And he certainly did. This week, he sold Mint.com for a cool $170 million to Intuit (NASDAQ: INTU).

Actually, over the past few years, I've been able to have calls as well as meet with Aaron. He gave strong demos and was always passionate about his company (he would even demo his personal account, showing that he would "eat his own dog food"). Mint.com seemed like a mission for him.

In a way, I wasn't surprised by the success of the website, which has attracted more than 1.4 million registered users.

OK, what can we learn from Aaron? Here's a look:

Solve a tough problem: In 2005, Aaron tried to use Quicken to do some budgeting. Unfortunately, it turned out to be a tedious process, as he had to go through bank statements and make various adjustments. Even though he had masters degrees in Computer Science and Engineering from Princeton and Duke, he still couldn't figure some things out.

He realized that millions of other people probably had the same frustrations. So that's when he got the inspiration for Mint.com: to create a free, easy-to-use web application that helps with budgeting.

To this end, he wanted to build something that could import financial information (in a secure way). He also realized that -- with this information -- he could use sophisticated technology to help people find ways to save money. Who wouldn't want that?

Skin in the game: For six months, Aaron lived off his personal savings and spent 14 hours a day building Mint.com. He also learned ways to bootstrap things, such as by using affordable web-based services and open source software.

Despite all the efforts, Aaron got many rejections from investors. Yet, this didn't stop him from continuing to build his product.

But, he finally found an investor -- First Round Capital -- that saw the potential for the idea and liked Aaron's passion. The result was a round of capital for $725,000.

Get experienced advisers: While it's common for founders to be adversarial with investors, this was not the approach for Aaron. He saw them as valuable allies -- in terms of helping get the next round of capital as well as much-needed advice.

For example, Aaron called First Round Capital's Rob Hayes on a frequent basis. He gave helpful advice on product development, usability testing, and building a team (you can read his blog post).

Market, market, market: Being a top company-builder is also about being a great salesperson. Aaron certainly is qualified. In fact, he spent 20% of his time on PR efforts (read Aaron's post on this).

While Mint.com spent some money on search advertising with Google (NASDAQ: GOOG), much of the marketing was grassroots. For example, the blog was extremely helpful in getting users to the site. There was also heavy use of social media, such as Facebook and Twitter.

Never enough: Recently, while talking to Aaron, I congratulated him on his success. He was thankful but said something interesting: "We have a long way to go."

Yes, this is always a great attitude when your building something new -- and trying to win the trust of customers.